ESS 202: Earthquakes

Lithosphere


The temperature at the earth's surface is near 0º C (freezing temperature of water).  The temperature increases rapidly to about 1500º C at 100 km depth.  From a depth of 100 km to about 2800 km, the temperature increases slowly by only 30º C per 100 km to a temperature of about 2500º C.   Then the temperature increases rapidly again by nearly 1000º C in 100 km to 3500 º C at the boundary between the mantle and core.   The cold rock of the upper 100 km is called the lithosphere .  Because it is cold it is very stiff.  This upper part of the Earth forms lithospheric plates.  These "plates" move around on the surface of the Earth as rigid blocks.   Most earthquakes are at the boundaries between the plates, where the plates are breaking.  Lithosphere responds to large stresses (force per unit area) as if it were a very stiff spring (elastic ).  If the stresses get too large the lithosphere breaks (brittle ).  Earthquakes are the brittle breaking of the lithosphere.

Between 100 and 200 km depth the rocks are hot and near their melting temperature.   Hotter rocks have lower viscosity .  These hot rocks behave like silly putty; they look like a solid, but given enough time they will flow.   The lithosphere behaves more like a pencil.   It will not flow.  It will bend if you apply a force to it.   If you push too hard it will break.   The cold lithosphere bends and breaks but does not flow.   The hot asthenosphere flows, but does not break.   Earthquakes occur in the lithosphere, not in the asthenosphere

There is no sharp boundary between the lithosphere and asthenosphere.   The Earth gets gradually hotter as you go deeper, so the viscosity of rocks gradually gets smaller.

On the other hand, the crust and mantle are different rock types.   For example the oceanic crust is mostly a rock called basalt, and the mantle is mostly peridotite.  Peridotite is denser.  There is a sharp boundary between the crust and mantle.   The boundary is called the Moho after its discoverer Mohorovicic in 1909.  Oceanic crust is only about 5 km thick, whereas the mantle is about 2900 km thick.   The lithosphere is the upper 100 km of the earth and includes the crust and the upper part of the mantle.   It varies in thickness from very small values in the Basin and Range to about 250 km under cratons such as the Canadian Shield.  The asthenosphere lies below the lithosphere and is about 100 km thick.  Beneath the asthenosphere the mantle gets gradually stiffer (more viscous), but remains viscous (flows very slowly). 

Except for very localized regions near active volcanos (e.g. Yellowstone, Mt. St Helens, Hawaii, mid-ocean spreading centers) the crust and mantle are both solid.
 

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